1. Field of the Invention
The present invention relates to an image recording apparatus in which a latent image is formed on an image holding material, a toner is selectively adhered to this to make a visible image, and then, it is transferred and fixed onto a recording member such as a sheet, and specifically to an image recording apparatus such as an electrophotographic recording apparatus, an electrostatic recording apparatus, an ionography, and an apparatus for forming an image using a magnetic latent image.
2. Description of the Related Art
Hitherto, as an image recording apparatus for reproducing an image signal or the like on a recording member such as a sheet, various systems of apparatuses have been put to practical use. For example, there is an apparatus in which a latent image is formed on an image holding material such as a photosensitive drum, a toner is selectively adhered to this to make a visible image, and this toner image is directly transferred to a recording member. There is also an apparatus in which a toner image is temporarily transferred onto an intermediate transfer material, and then, it is transferred onto a recording member.
The system in which an intermediate transfer material is used and a toner image is temporarily transferred onto this, is frequently applied to an apparatus for forming a color image. Toner images of multiple colors are superimposed and transferred onto the intermediate transfer material to form a full-color toner image, and this can be collectively transferred onto a recording member. Such a system has merits that mixture of toners of different colors stored in a developing unit can be prevented, and a full-color image can be formed in a short time by making a so-called tandem apparatus in which multiple image holding materials are provided. Moreover, in the image recording apparatus using the intermediate transfer material, when a toner is transferred from the intermediate transfer material onto the recording member, the toner is heated and melted, and the softened toner is pressed against the recording member, so that transfer and fixing can be carried out at the same time. That is, in the case where transfer is directly carried out to a recording member from an image holding material with a peripheral surface on which a toner image is formed, when the toner is heated and melted, a photosensitive material layer frequently used in the image holding material is also heated, so that its characteristics are changed and excellent image formation becomes impossible. However, when a toner image is temporarily transferred onto the intermediate transfer material and is further transferred onto the recording member, the influence of temperature upon the image holding material can be reduced, and the transfer and fixing can be carried out at the same time.
Incidentally, if an image holding material is not easily influenced by temperature as in ionography, the method in which a toner image is heated so that transfer and fixing are carried out at the same time, can also be used in the case where the image is directly transferred from the image holding material to a recording member.
There have been proposed some image recording apparatuses using the intermediate transfer material in which when a toner image is transferred from the intermediate transfer material onto the recording member, the toner image is heated so that transfer and fixing are carried out at the same time. Such an apparatus is disclosed in, for example, Japanese Patent Unexamined Publication No. Hei. 2-106774, No. Sho. 49-78559, No. Sho. 50-107936, and No. Sho. 57-163264, and Japanese Patent Publication No. Sho. 64-1027.
In the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 2-106774, a recording member is heated prior to transfer of a toner image onto an intermediate transfer material, and the toner on the intermediate transfer material is melted by the heat of the recording member, and is transferred and fixed onto the recording member.
In the techniques disclosed in Japanese Patent Unexamined Publication No. Sho. 49-78559 and No. Sho. 50-107936, a recording member is not heated, but a toner on an intermediate transfer material is heated by a radiation heating means up to its melting temperature, and the intermediate transfer material and the toner image softened on this are pressed against the recording member, so that transfer and fixing are carried out.
In the technique disclosed in Japanese Patent Unexamined Publication No. Sho. 57-163264, an intermediate transfer material and a toner image transferred thereto are previously heated, and in a state where a recording member is heated, both are pressed against each other, so that the toner image is transferred and fixed onto the recording member.
In the technique disclosed in Japanese Patent Publication No. Sho. 64-1027, toner is preliminarily heated before a nip portion (transfer and fixing region) where a toner image on an intermediate transfer material is pressed against a recording member. That is, a belt-like intermediate transfer material is wound around a heating roller at 90.degree. or more, and the toner is preliminarily heated before the nip portion by using the heat of the heating roller, so that the temperature is raised up to the vicinity of the melting temperature of the toner. Thereafter, the toner is further heated and melted at the nip portion, and the toner image is transferred and fixed onto the recording member.
However, the foregoing conventional techniques have problems described below.
The technique disclosed in Japanese Patent Unexamined Publication No. Hei. 2-106774 is preferable since the recording member is heated so that temperature rise of the intermediate transfer material is low and a bad thermal influence upon the image holding material is little. However, utilization efficiency of heat is low, and a large amount of heat energy is consumed for heating of the recording member. Especially in the case where image formation is carried out at high speed, it is necessary to increase the output of a unit for heating the recording member, so that the consumed electric power of the entire apparatus is increased. Besides, when interruption of conveyance of the recording member, a so-called jam occurs, since the recording member (generally, a PPC sheet) is heated to a high temperature, there is also a defect that the danger of firing is high.
The techniques disclosed in Japanese Patent Unexamined Publication No. Sho. 49-78559 and No. Sho. 50-107936 use a radiation heating system as means for selectively heating the toner, so that substantial thermal efficiency becomes low as compared with the heating means using thermal conduction such as a heating roller.
Since the technique disclosed in Japanese Patent Unexamined Publication No. Sho. 57-163264 heats any of the intermediate transfer material, the toner, and the recording member, there is a merit that the temperature of the intermediate transfer material can be set low. Besides, heat conduction between the toner image on the intermediate transfer material and the recording member at the press contact portion is low, and lowering of fluidity of the toner is lessened, so that the toner is sufficiently permeated into the recording member and is transferred from the intermediate transfer material. However, the temperature of the toner at the time when it is separated from the intermediate transfer material is higher than the toner softening point temperature, and the toner is in a fluid state, so that there is a tendency that the toner is divided and is apt to be offset to the side of the intermediate transfer material. Moreover, since any of the intermediate transfer material, the toner, and the recording member are heated, the consumed energy becomes high. Moreover, there is a problem that heat is conducted to the image holding material side by the circular movement of the intermediate transfer material heated by the heating roller, so that the temperature of the periphery of the image holding material is increased and the charging function is damaged. There also occurs a problem that the toner is melted in the vicinity of the developing unit by the temperature rise of the image holding material, or the toner is adhered to a cleaning blade or the like. On the other hand, in such a mechanism, when an attempt is made in order to prevent the conduction of heat of the intermediate transfer material to the image holding material side, a relatively large cooling apparatus comes to be required. Thus, the cost of the apparatus is greatly increased.
In the technique disclosed in Japanese Patent Publication No. Sho. 64-1027, since a toner is preliminarily heated before a nip portion (transfer and fixing region), the set temperature of the heating roller can be made low. However, since the toner and the recording member are again heated at the nip portion, the total energy required for fixing becomes large similarly to the foregoing technique.
As described above, in the image recording apparatus of the system in which toner images are temporarily transferred onto an intermediate transfer material and the toner images are collectively transferred onto a recording member and are fixed at the same time, any apparatus has some problems. The main problems of these are summarized into three points as follows.
The first problem is that when the toner images on the intermediate transfer material are collectively transferred onto the recording member at a secondary transfer portion, and at the same time, they are fixed by heating, the intermediate transfer material heated up to a high temperature is conveyed to a contact portion against the image holding material, so that the temperature of the image holding material is raised. When the temperature of the image holding material is raised like this, the charging characteristics, photosensitive characteristics and the like are changed, so that stabilization of images becomes difficult. Besides, there is also a problem that the toner is adhered to peripheral members through the temperature rise of the image holding material.
A second problem is that a large amount of thermal energy for melting the toner on the intermediate transfer material and for transferring and fixing it onto the recording member becomes necessary, so that consumed energy is increased. In general, thermal capacity of the recording member and the intermediate transfer material is large, so that a large amount of thermal energy becomes necessary to raise the temperature of those.
A third problem is that since the recording member is pressed in the state where the toner is heated and melted, when the recording member is separated from the intermediate transfer material, a part of the melted toner remains on the intermediate transfer material, that is, a so-called offset occurs. Although the offset can be reduced by using a material with good separability for the outer peripheral surface of the intermediate transfer material, when the temperature of the toner is high and its fluidity is high, the offset comes to be apt to occur.
On the other hand, as to the system in which the toner image transferred onto the recording member is fixed by heating, a technique for decreasing consumed thermal energy is disclosed in Japanese Patent Unexamined Publication No. Hei. 8-76620.
An apparatus disclosed in this publication uses a phenomenon that a magnetic field is applied to a heat generating member including a conductive layer so that eddy current is generated in the heat generating layer and the conductive layer having resistance is heated by this eddy current. That is, the recording member being in close contact with the heat generating member and the toner image held on the recording member are heated/melted by the heat generation of the conductive layer, so that the toner image is fixed onto the recording member.
By such a structure, consumed electric power for melting the toner is suppressed to a low level. However, since the toner and the recording member are together sandwiched between the heat generating member and the pressing roller and are heated, as a result, the consumed energy can not be reduced very much. Besides, since the toner is heated at the press contact portion between the heat generating member and the pressing roller, the temperature of the toner in the vicinity of the outlet of the fixing region, that is, of the press contact portion becomes high. Thus, there is also a problem that the offset is apt to occur.